1. Field of the Invention
The present invention relates to an organic electroluminescent element and method of manufacturing the same.
2. Description of the Related Art
Electroluminescent elements comprise at least a light-emitting material. Organic electroluminescent elements are elements constructed using organic compounds which emit light in response to electric signals.
Organic electroluminescent elements basically comprise an organic luminescent layer interposed between a pair of opposed electrodes. Luminescence is a phenomenon which occurs when a luminescent body within the luminescent layer is excited to a higher energy level and the excited luminescent body returns to its original base state and discharges said surplus energy as light. This is achieved by the injection of electrons from one electrode and the injection of holes from the other electrode.
In order to improve luminescence efficiency, the aforesaid basic construction supplemented by a hole injection layer added to the electrode that injects holes, and an electron transport layer may be added to the electrode that injects electrons.
An example of an electroluminescent element is disclosed in U.S. Pat. No. 3,530,325 which uses single crystals of anthracene in the luminescent element.
U.S. Pat. No. 4,539,507 discloses a combination of a hole injection layer and an organic luminescent layer.
U.S. Pat. No. 4,720,432 discloses a combination of a hole injection/transporting layer and an electron injection/transporting layer.
Organic electroluminescent elements having the aforesaid laminate layer constructions are constructed by superimposing, one over another, an organic fluorescent body, charge-transporting organic material (charge-transporting member), and electrodes, such that luminescence is generated when holes and electrons injected by the respective electrodes move within the charge-transporting member and recombine. Examples of material which are useful as organic fluorescent bodies include organic pigments which generate fluorescence such as 8-quinolinol-aluminum complex, coumarin compounds and the like. Examples of useful charge-transporting materials include N'-di(m-tolyl)N,N'-diphenylbenzidene, 1,1-bis[N,N-di(p-tolyl)aminophenyl]c yclohexane and like diamino compounds, 4-(N,N-diphenyl)aminobenzaldehyde-N,N-diphenylhydrazone compounds and the like. The use of porphyrin compounds such as copper phthalocyanine have also been proposed.
It has been reported that deterioration of luminescence characteristics over time can be suppressed, and the luminescence threshold electric potential can be reduced, by treating a positive electrode of Indium Tin Oxide ("ITO") film with a secondary process of plasma surface processing. This is one example of an enhancing agent which improves the practicality of organic electroluminescent elements (e.g., Institute of Electronics, Information, and Communications Engineers (IEICE), Spring Conference Proceedings, 5-347, 1993).
The term "plasma" as used herein means a completely ionized gas, composed entirely of a nearly equal number of positive and negative free charges (positive ions and electrons). The meaning of the term plasma as used herein is consistent with its definition in the McGraw Hill Dictionary of Scientific and Technical Terms, 1226 (1982).
The use of a plasma process as a method of washing an ITO film was disclosed in Japanese Laid-Open Patent No. 7-142168 and the like.
These plasma processes provide inadequate washing effectiveness. The plasma processing causes the excited atoms to impinge the electrode directly in a vacuum, and is harmful to the device. When a plasma processed substrate is used in the manufacture of electroluminescent elements non-luminescent areas of black spots occur within the luminescent surface, such that the luminescence starting potential is reduced and stable luminescent characteristics cannot be obtained.